© AS Jul-12

Electronegativity = the power of an atom to attract the electrons in a covalent bond

H 2.1 He Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0 Ar K 0.8 Ca 1.0 Sc 1.3 Ti 1.5 V 1.6 Cr 1.6 Mn 1.5 Fe 1.8 Co 1.8 Ni 1.8 Cu 1.9 Zn 1.6 Ga 1.6 Ge 1.8 As 2.0 Se 2.4 Br 2.8 Kr Rb 0.8 Sr 1.0 Y 1.2 Zr 1.4 Nb 1.6 Mo 1.8 Tc 1.9 Ru 2.2 Rh 2.2 Pd 2.2 Ag 1.9 Cd 1.7 In 1.7 Sn 1.8 Sb 1.9 Te 2.1 I 2.5 Xe Cs 0.7 Ba 0.9 La 1.1 Hf 1.3 Ta 1.5 W 1.7 Re 1.9 Os 2.2 Ir 2.2 Pt 2.2 Au 2.4 Hg 1.9 Tl 1.8 Pb 1.8 Bi 1.9 Po 2.0 At 2.2 Rn

Factors affecting electronegativity H – F

Factors affecting electronegativity 1)Nuclear charge – the more protons, the stronger the attraction from the nucleus to the bonding pair of electrons. 2)Atomic radius – the closer the bonding electrons to the nucleus, the stronger the attraction from the nucleus to the bonding pair of electrons. 3)Shielding – the less shells of electrons shielding (repelling) the bonding electrons, the stronger the attraction from the nucleus to the bonding pair of electrons.

Factors affecting electronegativity 1)Nuclear charge – more protons, stronger attraction between nucleus and bonding pair of electrons. 2)Atomic radius – closer to the nucleus, stronger attraction between nucleus and bonding pair of electrons. 3)Shielding – less shells of electrons between the nucleus and the electrons, less shielding (less repulsion), stronger attraction between nucleus and bonding pair of electrons.

Trend down a group Electronegativity decreases Atomic radius increases More shielding  Less attraction between nucleus and bonding pair of electrons

Trend across a period Electronegativity increases Atomic radius decreases More nuclear charge  Stronger attraction between nucleus and bonding pair of electrons

Formation of a covalent bond

Non-polar bond Polar bond

HCl polar bond showing the unequal sharing of a cloud of electron density. HCl polar bond showing the partial (  +) change on hydrogen and the partial (  -) change on chlorine. HCl polar bond showing the direction of the dipole with an arrow pointing toward the more negative atom. The + on the opposite end also reminds us which atom is more positive.

X YXY Y+Y+ X-X- Y+Y+ X-X- Electronegativity Difference 04 Pure covalent Polar covalent Electrons not equally shared Polar ionic Distorted ions Pure ionic Polarisation of covalent bondsPolarisation of ions Favoured by small, highly charged +ve ions, e.g. Li +, Be 2+ -- ++

Ionic bonding Covalent bonding

Difference in electronegativity decreases NaClMgCl 2 AlCl 3 SiCl 4 Mpt801ºC714ºC190ºC-70ºC StructureIonicPolar ionic Polar covalent Covalent +ve ion gets smaller and more highly charged, so –ve polarised more

Difference in electronegativity decreases BeCl 2 MgCl 2 CaCl 2 SrCl 2 BaCl 2 Mpt401ºC714ºC782ºC870ºC963ºC Structure Polar covalent Ionic +ve ion gets smaller and more highly charged, so –ve polarised more

O HH ++ ++ -- H2OH2O Bonds: polar Molecule: polar

N HH ++ ++ -- NH 3 Bonds: polar Molecule: polar H ++

CO ++ -- CO 2 Bonds: polar Molecule: non-polar O --

CCl 4 Bonds: polar Molecule: non-polar C -- -- ++ Cl -- --